Audio power amplifier for supplying electric power to a load by switching of power supply voltage

Abstract

A power amplifier in which a DC power supply is coupled to a load through a switching element, a smoothing circuit, and an output amplifying element. The output amplifying element receives an input signal voltage to be amplified. A voltage detecting circuit is provided which compares a feedback voltage proportional to an output volgage of the smoothing circuit with the input signal voltage to switch the switching element, thereby varying both the power supply voltage of output amplifying element and the load output voltage in accordance with the input signal voltage. As the output voltage of the smoothing circuit is compared with the input signal voltage, the delay of the change of the power supply voltage of the output amplifying element relative to the load output voltage can be reduced, and the generation of distortion and the lowering of efficiency can be suppressed.

What we claim is: 
     
       1. A power amplifier comprising: a DC power source;   a smoothing circuit;   a switching element connected between said DC power source and said smoothing circuit;   a load;   an output amplifying element which is an emitter follower transistor having its base connected to receive the input signal, its collector connected to said switching circuit and it emitter connected to said load; and   voltage detecting circuit means coupled to the output of said smoothing circuit and connected to receive the input signal voltage for comparing a feedback voltage from said smoothing circuit with the input signal voltage to switch said switching element, said voltage detecting circuit means having two different switching levels of the feedback voltage relative to the input signal voltage to switch said switching element from ON to OFF and from OFF to ON.   
     
     
       2. A power amplifier as claimed in claim 1, wherein said voltage detecting circuit means comprises means for adding a bias voltage to the input signal voltage. 
     
     
       3. A power amplifier as claimed in claim 1, wherein said voltage detecting circuit means comprises: a circuit means for producing the difference between the input signal voltage and the feedback voltage from said smoothing circuit;   a voltage comparing circuit means having a first input connected to receive the difference between the input signal voltage and the feedback voltage and a second input connected to receive a reference voltage, the output of said voltage comparing circuit being connected to a control input of said switching element; and   circuit means responsive to said voltage comparing circuit means for coupling to said first input of said voltage comparing circuit means a positive voltage having a predetermined magnitude when the output of said voltage comparing circuit means is at a first voltage level and a negative voltage having a predetermined magnitude when the output of said voltage comparing circuit means is at a second voltage level.   
     
     
       4. A power amplifier comprising: a DC power source;   a smoothing circuit;   a switching element connected between said DC power source and said smoothing circuit;   a load;   an output amplifying element connected between an output of said smoothing circuit and said load and having an input connected to receive an input signal voltage to be amplified;   voltage detecting circuit means coupled to the output of said smoothing circuit and connected to receive the input signal voltage for comparing a feedback voltage from said smoothing circuit with the input signal voltage to switch said switching element, said voltage detecting means having two different switching levels of the feedback voltage relative to the input signal voltage to switch said switching element from ON to OFF and from OFF to ON;   a second switching element connected between said DC power source and the output of said smoothing circuit; and   second voltage detecting circuit means connected to receive the input signal voltage and responsive to an abrupt change of the input signal voltage for turning ON said second switching element for a fixed period of time.   
     
     
       5. A power amplifier as claimed in claim 4, wherein said second voltage detecting circuit means comprises: a differentiation circuit for differentiating the input signal voltage;   a voltage comparing circuit for comparing an output voltage of said differentiation circuit with a reference voltage; and   a circuit connected to an output of said voltage comparing circuit for producing a control signal to turn ON said second switching element for the fixed period of time when the output voltage of said differentiation circuit exceeds the reference voltage.   
     
     
       6. A power amplifier as claimed in claim 4, further comprising: an active element connected between the output of said smoothing circuit and said output amplifying element and having an input connected to receive the input signal voltage.   
     
     
       7. A power amplifier comprising: a DC power source;   a smoothing circuit;   a switching element connected between said DC power source and said smoothing circuit;   first and second loads;   a first output amplifying element connected between an output of said smoothing circuit and said first load having an input connected to receive a first input signal voltage;   a second output amplifying element connected between the output of said smoothing circuit and said second load and having an input connected to receive a second input signal voltage;   a first voltage detecting circuit connected to receive the first and second input signal voltages for deriving from an output thereof either one of the first and second input signal voltages which is closer to the voltage of said DC power source; and   a second voltage detecting circuit connected to the output of said smoothing circuit and connected to receive an output voltage of said first voltage detecting circuit for comparing a feedback voltage from said smoothing circuit with the output voltage of said first detecting circuit to switch said switching element, said second voltage detecting circuit having two different switching levels of the feedback voltage relative to the output voltage of said first voltage detecting circuit to switch said switching elment from ON to OFF and from OFF to ON.   
     
     
       8. A power amplifier comprising: a first positive DC power source and a second negative DC power source;   first and second smoothing circuits each having an input and an output;   a first switching element connected between said first DC power source and the input of said first smoothing circuit;   a second switching element connected between said second DC power source and the input of said second smoothing circuit;   first and second complementary output transistors having their emitters coupled together, their collectors coupled to the outputs of said first and second smoothing circuits, respectively, and their bases connected to receive an input signal voltage to be amplified;   a load connected between said emitters of said first and second output transistors and ground;   a first voltage detecting circuit connected to receive the input signal voltage and connected to the output of said first smoothing circuit for comparing a feedback voltage from said first smoothing circuit with the input signal voltage to switch said first switching element, said first voltage detecting circuit having two different switching levels of the feedback voltage relative to the input signal voltage to switch said first switching element from ON to OFF and from OFF to ON; and   a second voltage detecting circuit connected to receive the input signal voltage and connected to the output of said second smoothing circuit for comparing the feedback voltage from said second smoothing circuit with the input signal voltage to switch said second switching element, said second voltage detecting circuit having two different switching levels of the feedback voltage relative to the input signal voltage to switch said second switching element from ON to OFF and from OFF to ON.   
     
     
       9. A power amplifier as claimed in claim 8, further comprising: a first diode having its cathode connected to the input of said first smoothing circuit and its anode grounded; and   a second diode having its anode connected to the input of said second smoothing circuit and its cathode grounded.   
     
     
       10. A power amplifier as claimed in claim 8, further comprising: a first diode having its cathode connected to the input of said first smoothing circuit and its anode connected to said second DC power source;   a second diode having its anode connected to the input of said second smoothing circuit and its cathode connected to said first DC power source; and   a bias circuit for applying an idling current to said first and second complementary output transistors.   
     
     
       11. A power amplifier comprising: a first positive DC power source and a second negative DC power source;   first and second smoothing circuits each having an input and an output;   a first switching element connected between said first positive DC power source and the input of said first smoothing circuit;   a second switching element connected between said second negative DC power source and the input of said second smoothing circuit;   first and second complementary output transistors having their emitters connected together, their collectors coupled to said outputs of said first and second smoothng circuit, respectively; and their bases connected to receive a first input signal voltage to be amplified;   a first load connected between emitters of said first and second complementary output transistors and ground;   third and fourth complementary output transistors having their emitters connected together, their collectors coupled to said outputs of said first and second smoothing circuits, respectively, and their bases connected to receive a second input signal voltage to be amplified;   a second load connected between emitters of said third and fourth complementary output transistors and ground;   a first voltage detecting circuit connected to receive the first and second input signals for deriving from an output thereof an input signal voltage which is closer to the voltage of said first DC power source;   a second voltage detecting circuit connected to outputs of said first voltge detecting circuit and said first smoothing circuit for comparing a feedback voltage from said first smoothing circuit with an output voltage of said first voltage detecting circuit to switch said first switching element, said second voltage detecting circuit having two different switching levels of the feedback voltage relative to the output voltage of said first voltage detecting circuit to switch said first switching element from ON to OFF and from OFF to ON;   a third voltage detecting circuit connected to receive first and second channel input signal voltages for deriving from an output thereof an input signal voltage which is closer to the voltage of said second DC power source; and   a fourth voltage detecting circuit connected to outputs of said third voltage detecting circuit and said second smoothing circuit for comparing a feedback voltage from said second smoothing circuit with an output voltage of said third voltage detecting circuit to switch said second switching element, said fourth voltage detecting circuit having two different switching levels of the feedback voltage relative to the output voltage of said third voltage detecting circuit to switch said second switching element from ON to OFF and from OFF to ON.